Oriented polymeric products

ABSTRACT

A continuous method of extruding an oriented plastic article of improved strength. A chemically reactive polymer, monomer or other compound is included in the matrix material(s) and a (multilayer) parison is extruded or melt formed having built-in thermoplastic orientation caused by shear or caused by elongational flow. A reaction can be activated in the molten state thereof by additional heat resulting in a cross-linked structure or at least the mobility of the molecules being reduced, resulting in a longer relaxation time which makes the freezing of permanent orientation possible. The soft matrix can be stretched while still hot in the hoop and/or axial direction. Then, the product thus obtain is calibrated and cooled in an oriented state.

This is a continuation of International Patent Application No.PCT/EP96/02801, with an international filing date of Jun. 26, 1996, nowpending.

BACKGROUND OF THE INVENTION

This invention relates to oriented polymeric articles, and moreparticularly to a novel oriented polymeric article comprising anoriented crystalline or semi-crystalline thermoplastic polymericmaterial having improved properties and a method and apparatus for itsproduction.

PRIOR ART

It is well known that the physical and mechanical properties ofcrystalline and semi-crystalline thermoplastic polymers can be improvedby orienting their structures. Polymer processing methods, such asdrawing, blow moulding, injection moulding and the like have all beenused to fabricate articles of thermoplastic polymers having orientedstructures.

In recent years, extensive studies have been directed to methods ofdeforming thermoplastic polymers in a solid state (ie. below thecrystalline melt temperature). In these methods, the polymer ismechanically deformed to obtain a desired uniaxial or biaxial molecularorientation. The polymer may be drawn, extruded or otherwise processedat temperatures ranging from the glass transition temperature totemperatures just below the crystalline melt temperature of the polymer.Products such as strip, tubes, rods and other shaped articles, usually,but not always, having predominantly unidirectional orientation, havebeen fabricated by such processing methods, for example, as described inU.S. Pat. No. 3,929,960 and U.S. Pat. No. 4,053,270.

Biaxially oriented containers, such as bottles used in the soft drinksindustry, are made by a melt extrusion-stretching or injectionmoulding-blowing expanding process. Such a process is described, forexample, in U.S. Pat. No. 3,923,943. The containers are produced bystretching the polymer, typically over 250 percent. Such largestretching deformations can result in non-homogeneous deformation of thestructure thereby damaging the spherulitic crystalline aggregates,causing the formation of microvoids and the enlargement of anymicrovoids already present in the polymer. The density of the polymer istypically decreased and the microstructural sensitive properties, suchas stress whitening and low temperature brittleness, remain.

Elongate, relatively thick-walled, high strength tubular polymerproducts, such as high pressure hoses, tubes and pipes have beenproduced by plasticating extrusion methods. One such method forproducing thermoplastic pipe is described in U.S. Pat. No. 3,907,961.The thermoplastic polymer is heated to a molten state and is extrudedwith a ram extruder through a conical shaped passage onto a flexiblemandrel. A cooling system for the die set is provided to cool thesurfaces of the pipe to a solidified state. The polymer is extruded inthe molten state and the resultant pipe has an unoriented structure.There is no mentioning of the use of cooling for enhancing orientation.

A further method for producing high pressure pipe is described in U.S.Pat. No. 4,056,591, which is directed to a process for controlling theorientation of discontinuous fibre in a fibre reinforced productproduced by melt or plasticating extrusion. The fibre-filled plasticsmatrix is extruded through a diverging die having a generally constantchannel. The walls of the die may taper slightly so that the area of theoutlet of the die is larger than the area of the inlet of the die. Theamount of orientation of the fibres in the hoop direction is directlyrelated to the area expansion of the channel from the inlet to theoutlet of the channel. The product is a reinforced pipe containingfibres that are oriented in the circumferential direction to improve thecircumferential properties.

While the fibres may be oriented, the polymer is substantiallyunoriented, since it is processed in a molten state. In other words,because the fibre reinforced polymer is processed in a molten state, thestructure is not composed of platelet or wafer-like, radially compressedspherulitic crystalline aggregates highly oriented bothcircumferentially and axially, although the fibres added to the polymermay be oriented circumferentially and axially.

A typical method for the production of oriented polymeric pipes, forexample, PVC pipes, is set forth in WO90/02644. The method includes thesteps of continuously effecting an initial extrusion of a tube,temperature conditioning to a desired orientation temperature, expandingthe tube by pressure within an internal pressure region and cooling.This process relies on stretching the polymeric material after leavingthe die set at orientation temperature, typically 95 deg. C. for PVC.The drawback is the substantial line length required for the temperatureconditioning operation.

A method of orienting polymeric pipes, for example, PVC pipes, using aninternal mandrel, disposed outside the die set, is described in DE2357078. This method also relies on temperature conditioning the pipe toa suitable orientation temperature where the stretching causesorientation of the plastics material.

Still another method which also relies on attaining a suitableorientation temperature is set forth in JP 4-19124. In this method thestart up is performed in a radially expanding closed die but the diecasing is removed when the orientation temperature, lower than theextrusion temperature, is reached.

Several methods for orientation of the material of a polyethylene pipehave been proposed but none of them has come into commercial use so far.Polyethylene is a highly crystalline material which can be successfullyoriented below its crystalline melting point only by solid stateextrusion or by using very high stretching forces in a die-drawing batchprocess. Above the crystalline melting point, orientation can beeffected during extrusion of the pipe, but only in a very narrowtemperature range. A great problem in this case is that the orientationdisappears rapidly, and that only thin-walled products can be cooledrapidly enough to maintain the orientation. An example of a typical linearrangement for producing a thin walled polyethylene shrinkable pipe isdescribed in EP 0507613.

Deformation of cross-linked polyethylene pipes is known, for example,from several patents mainly dealing with heat shrinkable products. Forexample, DE 2051390 describes a method of continuous manufacture ofpipes formed from cross-linked polyolefins wherein the completelycross-linked material is reheated after leaving the die set, expanded,and then cooled in the expanded state. The expansion is affected bymeans of a mandrel. There is no mention of the degree of expansion andalso no mention of orientation being effected by the expansion. Thedegree of cross-linking after expansion is not stated. The method isused for the manufacture of shrinkable tubes.

DE 2200964 describes a method for the production of cross-linkedpolymeric tubes. Typically, the cross-linking is started in the extruderhead, or after exiting the die set.

DE 2719308 describes a method for the manufacture of shrinkable tubeswhere the crosslinking is initiated after the die. Orientation is notused for increasing the strength of the product. EP 0046027 describesanother method for the manufacture of cross-linked shrinkable products.

U.S. Pat. No. 3,201,503 discloses a method for the production ofcross-linked shrinkable films. In this method the molten polymercontaining a peroxide is extruded in a separate cross-linking chamberand then blown into a larger diameter tubular member. The extrusion ofcross-linked hot water conduits is mentioned but these conduits are notoriented

EP 0126118 describes a method for the orientation of a plastics pipewherein the pipe after leaving the die-head is passed through a heatedhollow jacket in order to cross-link the material, and wherein thecross-linked pipe inside the jacket is expanded after cross-linking byinternal pressure to engage the inside of a wider portion of the jacket.There is no mentioning of the extrusion temperature or of the additionof cross-linking agents, and no disclosure of axial orientation andcooling of the plastic pipe. The process also requires a long temperingtube as the pipe is essentially heated by a heat flow coming from theoutside jacket only.

GB 2089717 describes an extruder for manufacturing plastic pipes with anelongated torpedo fixed to the screw end or mounted through the screw.The aim is to avoid the adverse effect of spider legs in the tooling.The patent mentions orientation but does not describe how the methodwould be able to produce permanent orientation in the product. Theunderlying concept is to utilize the internal shear from an internalrotating mandrel and the external shear caused by material flow axially(nowhere is it mentioned that the flow can be plug flow). Although theuse of cross-linked polymers is mentioned in the patent there is nosuggestion that cross-linking would enhance orientation. There is noinformation either as to where in the extruder the cross-linking wouldtake place. The aim is to obtain a hot water pipe having an outersurface with less cross-linking in order enable welding to take place.

Orientation using a smooth mandrel is also known from EP 0563721. Inthis method the parison is driven over the mandrel by using acorrugator. Although the drawing shows a conical mandrel before theorientation mandrel there is no mention of any benefits of thisarrangement. The mandrel is simply used in order to bring the parisoninto contact with the mould blocks. Also the process is based onstretching the parison after it exits the closed area of the die set.

Patents having disclosures relating to the manufacture of pipes, and/orthe manufacture of composite metal/plastics pipes include, for example:

Swiss Patent no.434716, U.S. Pat. No. 4,144,111, DE 2606389, FR 1385944,Swiss Patent no.655986, EP 0067919, EP 0353977, DE 3209600, EP 0024220,U.S. Pat. No. 3,952,937, GB 2111164, DE 2923544, DE 2017433, DE 1800262,DE 2531784, DE 2132310, and EP 691193.

The disclosures of all the abovementioned patents are incorporatedherein by reference in their entirety and for all purposes.

The prior art extrusion processes described above, by which tubularproducts consisting essentially of thermoplastic polymers are produced,are incapable of and cannot be adapted to expand a polymer by at least100 percent in the circumferential direction in a compression-typedeformation. Prior art processes for producing hoses or elongatedtubular products are directed to melt or plasticating extrusionprocesses that generally result in the production of non-orientedproducts.

Prior art processes for producing large diameter containers are directedto stretching or tensioning processes in which a polymer is expanded atleast 100 percent in the circumferential direction. Stretching ortensioning causes non-homogeneous deformation of the spheruliticcrystalline aggregates in the polymer structure. The spherulites areruptured and tilted. Microvoids, microfibrils and eventually fibrils areformed. Defects, such as microvoids already present in the polymer areenlarged. The resulting products are highly oriented in acircumferential direction, but have defects formed in the structure.

OBJECTS OF THE INVENTION

It is an object of the invention to produce an article comprising acrystalline or semi-crystalline polymeric material that is permanentlyoriented at ambient temperatures.

It is a further object of the invention to provide a deformation methodthat is compressive in nature whereby the problems of non-homogeneousdeformation and the associated product defects are substantiallyobviated and an oriented spherulitic crystalline aggregate structuresubstantially free from such defects is obtained.

It is a still further object of the invention to provide an articlecomprising a crystalline thermoplastic polymeric material which issubstantially free from defects caused by non-homogeneous deformation ofthe polymer, is oriented in both a circumferential direction and anaxial direction, and has particularly improved circumferential burststrength and tensile impact strength over the ambient to low temperaturerange, and substantially retains the density of the polymer from whichit is processed.

It is a yet further object of the invention to provide an articlecomprising a crystalline thermoplastic polymeric material which isexpanded at least 100 percent in the circumferential direction and isexpanded at least 50 percent in the axial direction, has a structureconsisting essentially of discrete platelet or wafer-like, radiallycompressed, spherulitic, crystalline aggregates which are oriented inboth the circumferential and axial directions, is substantially free ofprocess-induced defects, such as microvoids, and has a density which isthe same as or higher than the same polymer when processed into anarticle by prior art processes and which has an improved circumferentialtensile impact strength and is less susceptible to furthermicrostructural damage on subsequent stretching.

Other objects of this invention will appear more clearly from thefollowing description and Drawings.

BRIEF DESCRIPTION OF THE INVENTION

In a first aspect, the invention provides an article comprising acrystalline or semi-crystalline thermoplastic polymeric material whereinthe polymeric material is cross-linked, or has grafted side chainscreating steric hindrance, and is permanently oriented at ambienttemperature, such that the tensile strength of the polymeric material inthe direction or directions of orientation is greater than the tensilestrength of an unoriented article formed from the same polymericmaterial.

In a second aspect, the invention provides a method for the productionof an oriented crystalline or semi-crystalline thermoplastic polymericarticle which comprises:

(i) heating a crystalline or semi crystalline thermoplastic polymericmaterial to a temperature at or above its crystalline melting point;

(ii) forming the polymeric material into an article whilst at or aboveits crystalline melting point;

(iii) subjecting the polymeric material to shear forces and/orstretching either during or after the formation of the article to effectorientation of the polymeric material in the longitudinal and/ortransverse directions;

(iv) reacting the polymeric material either before, during or after theformation of the article, either before or during orientation, or afterorientation but before substantial relaxation of the orientation hastaken place, with a cross-linking agent, or with a grafting agentwhereby steric hindrance of polymer chain movement is increased;

the resultant article having a tensile strength in the direction ordirections of orientation greater than the tensile strength of anunoriented article formed from the same polymeric material.

In a third aspect, the invention provides an extrusion apparatus forproducing an oriented extrudate of polymeric material, comprising:

a) a plasticating extruder means for providing a melt(s) or partialmelt(s) of said polymeric material(s) and a chemically reactivesubstance and for feeding said melt or partial melt under pressurethrough a discharge opening in said extruder means;

b) an elongational flow pattern-developing cavity means having an inletopening communicating with said discharge opening of said extrudermeans, a flow cavity, and a discharge orifice, the relative geometriesof said flow cavity and said discharge orifice being such as to developwithin the molten polymeric material flowing from said extruder meansthrough said cavity means an elongational flow pattern which inducesmolecular orientation at least transversely to the direction of flowwithin said molten polymeric material;

c) an orientation-retaining extrusion die means provided with an orificeextending therethrough, said die orifice having an inlet end and anoutlet end, said discharge orifice of said cavity means opening intosaid inlet end of said die orifice so as to permit flow of the orientedmolten polymeric material from said cavity means into said die means,said discharge orifice having a cross-sectional area in the range offrom 0.9 to 2.0 times the cross sectional area of said die orifice;

d) temperature control means for maintaining the temperature of saidflowing molten polymeric material below the reaction temperature of thesaid chemically reactive substance in the extruder and in at least afirst part of said cavity means, and for maintaining the temperature ofsaid flowing molten polymeric material above said reaction temperaturein at least a second part of said cavity means and/or said inlet end ofsaid die orifice;

e) optionally, temperature control means for maintaining in said dieorifice an axial temperature gradient descending in the direction offlow through a median die temperature substantially equal to the normalmelting point of said polymeric material so that solidification of saidpolymeric material will be inhibited in the entrance region of said diemeans and may be initiated within said die means;

f) optionally, a variable speed take-up means for withdrawing anextrudate of said polymeric material from said outlet end of said dieorifice at a controlled draw rate;

the arrangement being such that said extrudate begins to solidify withinsaid die means or after exiting from said outlet end of said die orificebut before any substantial radial swelling of said extrudate can occur.

The invention is particularly applicable to the production of hollowarticles, especially elongate hollow articles such as pipes, tubes,conduits and the like, and will be more particularly described hereinwith respect to the production of such articles. It is to be understood,however, that the invention is not limited to the production of sucharticles and may find application in the production of bottles,containers, rods, wire and cable coatings, pipe fittings and otherpolymeric articles.

In this specification, the crystalline melting point of the polymericmaterial is define as the temperature at which crystals start to form oncooling the polymeric material from the melt and can be determinedaccording to the method of ASTM-D648.

The invention is based in part on the realisation that in order for thehigh molecular orientation, developed within the polymeric material, forexample, by its elongational flow through an extrusion die, or bystretching, to be retained in the final product, it is necessary to`freeze` such orientation by solidifying the polymeric material beforethe oriented molecules have sufficient time to relax. Because of the lowthermal conductivity of polymers, combined with the short relaxationtimes of most molten plastics materials flow-induced orientation cannotnormally be locked into the final structure to produce articles withsignificantly improved values of modulus and strength.

In the method of the invention a chemically active substance, which canbe a reactive polymer, monomer, or other suitable compound is added tothe polymeric material in order to facilitate orientation of thematerial and thus provide a method of orientation that is suitable forpracticable commercial use. The chemically reactive substance can, forexample be a cross-linking agent, a grafting agent, or a reactivecompound which can add bulky end groups to the polymer molecules.

The addition of such chemically active substances, for example,peroxides for cross-linking of polyethylene, is known per se in order toobtain cross-linking of the material in extrusion of hot water pipes.Typically such pipes, made of ultra high molecular weight polyethylenes,are cross-linked to a gel level of 60 to 80% in an attempt to achievegood quality and low creep properties at elevated temperatures. However,even if radial expansion of the pipe has been effected in connectionwith the extrusion, the sole purpose of adding a cross-linking agent hasbeen to obtain cross-linking and thereby enhanced creep properties atelevated temperature.

Now, it has been surprisingly discovered that an addition even at verylow levels of a cross-linking agent has a dramatic effect on theorientability of plastic materials. For example, when extruding andorienting polyethylene pipes at 200 degrees there would be no way ofreaching permanent orientation because the stress in material wouldimmediately relax away. However, with a slight cross-linking of thepolymeric material to a degree of 1 or 2% or more, preferably 10-20%,before orientation, we have found that there is still a considerable,for example, more than 50%, enhancement of the strength properties ofthe product after orientation. Similar effects are obtainable bygrafting bulky side chain molecules onto the polymer chains as will bedescribed hereinafter. In this specification, the degree ofcross-linking is expressed in terms of gel content, as measured byANSI/ASTM D2765-68.

The addition of a cross-linking agent before or during the extrusion ofa polymeric material in order to facilitate orientation is alsodescribed and claimed in our co-pending Swedish patent application no.SE 9503272-8, the entire disclosure of which is incorporated herein byreference for all purposes.

In comparison with the prior art technology, where the extrusiontemperatures have been well below 150 degrees, in the present invention,the temperature range over which orientation of the polyethylenematerial can be achieved is dramatically broadened: the feasibletemperatures in this process typically range from 135° C. to 250° C.,the preferred process temperature being around 180 deg.C for practicalreasons.

Even more interesting, is that the invention has been found to beapplicable to many different polymeric materials. Formerly, thepolyethylene (PE) grades that could be oriented were special, highpriced polymers with specific molecular weight distribution andcomparatively high molecular weight. The method of this inventionenables a much greater variety of polymers to be used. For example lowcost LDPE when partly cross-linked and mixed with high molecular weightPE will enhance dramatically its orientation capabilities even at lowconcentrations.

In a further embodiment, the invention provides a reactive extrusionmethod, if necessary with appropriately adapted extruders or conicaldies, which leads to improved orientation in the polymer matrix whereinthe immobilization of the molecule chains is achieved not bycross-linking but by grafting reactions or end group addition, wherein aside or end group of considerable size is joined to the chain. Thesteric hindrance thus obtained improves the orientation properties ofthe matrix. This will no doubt give interesting possibilities especiallyin the field of biopolymers. Preferred grafting reactions are, forexample, melt free radical grafting reactions using monomers capable ofintroducing bulky side groups. Suitable monomers can comprise, forexample, oxazoline groups, and a specific example is ricinoloxazolinemaleinate (OXA). The grafting reaction, using polypropylene as anexemplary crystalline thermoplastic polymeric material, is illustratedin FIG. 6 herewith. In this reaction, the degradation to b-scission ispreferably minimised, for example, by the addition of suitable quinonesor other means. It will be apparent that by varying the size of thegroup R the properties of the polymeric material and the effect onorientability can be optimised. The percentage of reacted side or endgroups joined to the polymer chain can vary from 1 to 100% as desired.

During the development of the new polymeric article another phenomenonwas unexpectedly discovered. If the mobility of the molecule chains canbe reduced, a stronger oriented product can be obtained. For example, ithas been found that the addition of fibre-like additives to the polymermatrix increases the product strength more than would be evident innormal technologies (without subsequent molecular orientation). Withoutwishing to be bound by any particular theory, it is believed that thefibres, especially when using the novel extrusion techniques describedelsewhere in this specification, tend to immobilize part of the polymermatrix thus forcing an additional molecular orientation to be generatedin addition to the fibre orientation. Somehow these fibres may act aseffective nucleating agents that bring a favourable structure to thepartly crystalline oriented matrix.

In a preferred embodiment of the invention, it has further been foundthat by inducing shear orientation to the matrix or by inducing drawover the cross-section before the cross-linking or grafting takes place,the polymeric material can become even stronger. It is believed thatthis embodiment of the invention, where the molecules are pre-alignedwith thermoplastic orientation before cross linking or grafting, whenused for manufacturing oriented thick walled products, brings out thestrength of the molecule chains better than in the case where the matrixis cross-linked or grafted in a random state (i.e. where the chains arecross-linked freely) and it seems that the carbon-carbon strength of thealigned chain can be greater than the strength of bonds achieved bynormal cross-linking.

Finally, although the invention is not limited to any particular theory,it is thought that the effect of cross-linking or grafting onorientation is basically related to the principle of using well adheredfibres as an immobilization vehicle for the matrix. The cross-linksprobably work as `in situ` fibres.

This basic principle means that in the present invention it may also bepossible to get better than predicted oriented products by using liquidcrystal plastics (LCP) in the matrix in order to enhance molecularorientation. Also, for example, it may be possible to blend lowviscosity PE impregnated with a cross-linking agent with higherviscosity PE, and extrude the mix from an extruder giving a helicaldistribution of the mass, with the result that in the final orientedproduct there is produced an interlacing orientation field ofcross-linked molecules stemming mostly from the LDPE embedded in apartially oriented matrix.

The novel polymeric articles of the present invention are permanentlyoriented at ambient temperature, which is to say that the orientation issubstantially retained unless the temperature of the article issubsequently raised to an elevated temperature at which polymer chainmobility again becomes evident. The amount of orientation in theplastics material can be detected by any suitable method, for example,by infra-red spectrophotometry combined with a wire grid polarisator.The results of the measurement of absorption peaks can be mathematicallyanalysed and a feedback can be connected to a process control system forthe extrusion apparatus, for example, the control system for theextruder and heating units. Thus it is possible to arrange for theorientation of the polymeric material to be controlled by an automaticprocess control system.

Other novel effects can be obtained using certain embodiments of themethod of the invention. A desired balance between axial draw anddiametral draw can be readily mastered in this process. Normally inorientation the measures for effecting this balance are limited.Controlling extrusion or haul off speed is one possibility, but thiseasily leads to unnecessary high orientation in the axial direction.

In this specification, axial draw ratio is defined as:

    ______________________________________                                        new length after drawing /                                                    original length × square root of diametral draw ratio,                  and                                                                           diametral draw ratio is defined as:                                           new mean diameter /                                                           original mean diameter.                                                       ______________________________________                                    

The method of the invention can be used, for example, to produce noveloriented thermoplastic pipes having controlled degrees of biaxialorientation in the axial and hoop directions, and more particularlyhaving a tensile strength measured in the hoop circumferential directionwhich is, for example, at least twice the tensile strength measured inthe axial direction. This combination represents the optimum combinationfor burst strength of unconfined pressure pipe. The process of theinvention, however, gives almost unlimited control possibilities. Forexample, when using initial shear induced orientation, one is able toproduce a `feed stock` or parison containing primarily totally hoopdirected molecules. When this parison is then (further) cross-linked andexpanded onto a mandrel, a product with enhanced radial orientation isobtained. Now, when exiting the die-set, the haul off speed can beadjusted so that the hoop orientation opens up in the axial direction,generating a net-like interlaced orientation structure easy to balanceto obtain the desired properties. The specific process needs orlimitations no longer dictate the product properties and optimisedproperties can be achieved. For example, when a closed die system isused the polymeric material can be pushed over a mandrel and no axialdraw is necessary.

In a particularly preferred embodiment of the method of the invention,the polymeric material is oriented in a plurality of stages, which can,for example take place before and after cross-linking or grafting. Inanother aspect, accordingly, the invention provides a method of formingand continuously orienting a product comprising polymeric material(s) ata temperature greater than the crystalline melting temperature of saidmaterial(s), characterized by the steps of:

adding a chemically reactive substance(s) to the polymeric materialbefore or during forming of either the entire product, or to one or morelayers of a multilayer product, or to axial or helical stripes of theproduct, or to certain segments of the product in the axial direction;

plasticizing and forming a parison of the polymeric material(s) thusprepared at a temperature not high enough to activate the reaction ofthe said reactive substance(s);

optionally, inducing shear at least to the layer(s) where the chemicallyreactive substance(s) have been added to and/or stretching the stillsoft parison in one or both of two directions, simultaneously orstepwise, said stretching including axial draw to effect thermoplasticorientation of the material in the longitudinal direction of the parisonand/or radial expansion to effect thermoplastic orientation of thematerial in the hoop direction of the parison;

decreasing the mobility of the molecules in the layer(s) to be orientedby activating a chemical reaction between the chemically reactivesubstance(s) and the polymeric material(s) having the chemicallyreactive substance(s) added thereto when the polymeric material(s) stillis/are in a molten state during extrusion and allowing the reaction(s)to proceed to a degree ranging from 1,0 to 100 % calculated from thenumber of chemically reactive groups;

inducing shear at least to the product,layer(s), stripes, or segments towhich the chemically reactive substance(s) have been added and/orstretching the still soft, at least partly reacted parison in one orboth of two directions, simultaneously or stepwise, said stretchingincluding axial draw to effect orientation of the material in thelongitudinal direction of the parison and/or radial expansion to effectorientation of the material in the hoop direction of the parison;

calibrating and cooling the parison in the oriented condition to makethe orientation permanent at least in the layer(s) where the chemicalreaction(s) has taken place.

In another particularly preferred embodiment of the method of theinvention, the polymeric material can be subjected to a furthercross-linking in a further cross-linking stage after the initialorientation and cross-linking or grafting has taken place. It has beenfound that, whereas a degree of cross-linking of from 1 to 80%,preferably at least 2 to 80% is enough to increase the orientationtemperature range sufficiently in many instances, further cross-linkingof from 99 to 20% can be effected in order to improve dimensionalstability still further.

The further cross-linking can be carried out, for example, byirradiation, using gamma radiation or electron beam radiation.Preferably, however, the further cross-linking is carried out byactivation of residual cross-linking agent in the polymeric material,for example, by heating. The residual cross-linking agent activated inthis way can be a remaining portion of a cross-linking agent involved inthe initial cross-linking reaction, or another cross-linking agent whichis activated at a higher temperature. The further cross-linking need notnecessarily be carried out at the time of manufacture of the polymericarticle. For example, the further cross-linking could be carried outafter a pipe has been laid and bent into a desired shape. In this case,further cross-linking could be carried out, for example, by re-heatingthe pipe by means of an electrical heater, which could be incorporatedinto the pipe as a conductive metal or plastics layer duringmanufacture. The electrical heater could be arranged to activateresidual amounts of cross-linking agent, for example, a peroxide,deliberately left in the polymeric material of the pipe.

In a still further aspect of the invention, the process can be used togive novel products having very interesting new properties. Because theorientation can be `activated` in any part or layer of the product, forexample, with the help of magnetic, dielectric or microwave inducedheating, products having specific properties can be engineered.

For example, tubular products with an inert inner layer or internalwall, a chemically cross-linked load bearing oriented middle layer and aradiation or photoinitialized cross-linked outer layer can be formed.Likewise the manufacture of three layer pipes with cross-linkedpolyethylene (PEX)-foam in the middle layer and an oriented media pipeinner layer becomes feasible with this new technique. Both physical andchemical foaming agents can be used, as appropriate, and after orientingthe body of the polymeric material, the layer comprising the foamingagent can be expanded to an extent controlled by any internal andexternal cooling after leaving the die set and by any calibration unitused.

Also, by using, for example, a process similar to that described inWO90/08024, if the central mandrel is made conical, and force is appliedto the extruded product , as described, for example, in WO93/25372,oriented pipe bends can be produced.

According to yet another aspect of the Invention, the manufacture ofother novel hollow products, for example pipes, with tailor-madeproperties is facilitated. The product to be manufactured can be, forexample, a composite product such as a multilayer pipe wherein thelayers may be of different plastic materials, or a pipe with axialstripes of different plastic materials. The layers or stripes can becross-linked or non-crosslinked, and where they are crosslinked they caninclude different cross-linking agents. The expression `differentmaterials` also includes materials of the same chemical composition butcrosslinked to different degrees ranging from 0 to 100%.

By the addition of a cross-linking agent to only that section of theproduct that is to be oriented, products with greatly varying propertiescan be made, such as products wherein, for example, an inner layer ismade of non-oriented material to have better abrasion resistance, whilean outer layer of pigmented non-oriented material can be advantageousdue to better welding properties.

In a further aspect of the present invention, elongate, compositetubular articles such as pipes can be produced comprising an orientedcrystalline or semi-crystalline polymeric layer and a tubular layer of adifferent material, for example, a metal layer.

The tubular layer of different material can be pre-formed, for exampleby extrusion, or formed in situ by helically wrapping a sheet or stripof the material and welding, for example, by continuous butt welding orultrasonic welding, or mechanically interlocking, the adjacent edgeregions. Where the different material comprises a metal pipe which isformed in situ, the metal sheet or strip can be formed into a pipeadjacent to the extruder orifice, so that the polymeric material isextruded inside an already formed metal pipe. For example, an orientablepolymeric material may be melt extruded in an extrusion apparatuscomprising an annular orifice having a diametrically diverging geometrywhereby the molten polymeric material is circumferentially oriented andpressed against the inner wall of the metal pipe or tube, for example,by using a mandrel. Alternatively the metal strip can be helicallywrapped around the extruded oriented polymeric material pipe, forexample, by rotating the extruded pipe. In the latter case, it may benecessary to support the extruded pipe on a mandrel, which can also beused to expand and orientate the polymeric material.

A suitable material for forming the metal pipe or tube is aluminiumfoils which can have a thickness ranging, for example, from 0.2 to 5 mm.Preferably the metal is coated with an adhesion promoter. The innersurface of the metal strip or sheet is preferably also roughened orserrated in order to improve the adhesion properties. If desired, it isalso possible to use corrugated sheet or strip to form the wound metalpipe.

Where the different material comprises a pre-formed metal pipe, the pipecan act as a heat sink to conduct heat away from the oriented plasticsmaterial layer more quickly and assist in retaining the orientationthereof.

The method of the invention can be applied advantageously, for example,to the method for producing multilayer metal composite hollow articlesas described and claimed in our co-pending International patentapplication no. PCT/FI96/00359, the entire disclosure of which isincorporated herein by reference for all purposes.

In a further embodiment of the invention, a composite tubular articlecan be formed by extruding the plastics material over an elongate membercomprising a different material, for example, a tubular member such as ametal pipe, or a solid core, for example a metal cable. In thisembodiment also, the metal pipe or cable can act as a heat sink, coolingthe extruded plastics material as it comes into contact with the pipe orcable.

Where the polymeric material is extruded into contact with a metal pipeor tube, the polymeric material can then be oriented, or furtheroriented, by transporting the pipe or tube at a speed greater than theextrusion speed, thereby imparting an axial draw to the extrudedpolymeric material. The axial draw can be, for example, of the order of100 to 400%, and further external cooling can be provided as required.

Where the metal layer is the outer layer, it can be protected by coatingwith another extruded layer of polymeric material, for example using afurther extrusion line and an offset die. The extruded outer coating ofpolymeric material is cooled by and adheres to the metal layer, and canalso be drawn so that the coating forms a strong axially orientedpolymeric outer layer.

Similarly, elongate, composite tubular articles comprising an inner orouter layer of oriented plastics material and a different materialcomprising a fibrous layer, a plastics layer with fibre reinforcement,or a composite layer comprising multiple layers of aluminium andplastics material can also be produced.

Composite metal pipes as described above, utilising the combinedstrength and physical properties of the metal layer and the orientedpolymeric material layer, can possess a very high hydrostatic strength,and can have very high permeation resistance and excellent impactstrength. When combined with a foamed insulation layer as describedherein, these properties can make them especially suitable for largebore oil and gas applications. For example, they are especially usefulin high pressure trunk lines operating up to about 60 bar. The combinedring stiffness of the metal and oriented polymeric material layers canenable the pipe to respond elastically to large deformations, forexample due to soil stress, without failure.

Although it is possible using the method of this invention to produceoriented pipes which are stable at both ambient and elevatedtemperatures (i.e. not heat shrinkable), in another aspect the inventioncan be used for the manufacture of heat shrinkable articles withinteresting properties. Such articles are stable at ambienttemperatures, but when raised to an elevated temperature they assume anew shape. For example, in a multilayer pipe having layers of differentmaterials the layers may have different shrinkage properties, whichmakes the pipe behave in a unique way when heated, especially if arotating die technology has been used. For example, if a pipe has anoriented outer layer of crosslinked polyethylene, (PEX), and an innerlayer of non-crosslinked polyethylene, (PE), the composite pipe willbend slightly if heated above the glass transition temperature (Tg),depending on i.e. relative wall thickness and centring of the layers.Also, the inside PE layer may assist in preventing the whole bent pipefrom losing internal diameter when heated if made strong enough by theuse of fillers.

The incorporation of fillers into at least the non-crosslinked layer ofa multilayer product is often beneficial because the improved thermalconductivity improves cooling and increases the possibility ofpreventing fast relaxation, hence making permanent orientation easier toachieve.

In general, the incorporation of fibres can very effectively stop thePEX tendency to shrink back (relax), which also makes postformingoperations like socketing of pipes easier. Hence it can be seen that apipe that is fibre reinforced, crosslinked and oriented offers anoptimized set of properties needed for a variety of piping applications.The inclusion of fibres into highly viscous olefin (co)polymers is notvery easy, and therefore a separate layer of softer material, whereinthe blending can more readily be done, is sometimes highly beneficial.

A suitable method for producing an article comprising a polymericmaterial comprising oriented fibres is described and claimed in ourco-pending Finnish application no. FI 960768 the entire disclosure ofwhich is incorporated herein by reference for all purposes.

Compared to non-oriented homogenous pipes that exhibit the same modulusin all directions, the oriented pipes of the invention are already animprovement because, for example, by varying the draw directions andratios, the hoop strength can be easily double the axial strength, acommon requirement in pressurized pipelines. By adding fillers thepossibilities to build up the strength of the composite becomemultiplied. This is especially true for flake-like fillers like mica,for example, which exhibit better than normal barrier properties whenembedded in a cross-linked structure.

A multilayer product having an inner non-oriented layer and an orientedPEX outer layer can also give interesting properties if, for example,the inner layer has a higher melting point than the softening point ofPEX, which is around 130° C. The inner material could be, for example, apolypropylene (PP) grade, which additionally shows very suddensoftening. This combination could be used as a fast shrink and/orelectrofusion sleeve that additionally can generate high shrinkingforces. Adhesion between the inner layer and the outer layer can beachieved, for example, by using an intermediate adhesion layer betweenthe inner and outer layers. A suitable adhesion layer can comprise, forexample, a blend of a PE and a PP having substantially the same meltingpoints together with a compatibiliser.

The use of non-crosslinked material surface layers on both sides of theoriented product can greatly improve the orientation process becausethese layers can be used to minimize friction against the tooling. When,for example, silicone oil is mixed only with a thin surface skin layerit will not substantially disturb the cross-linking process and theconsumption thereof is greatly reduced compared to mixing with the wholebulk of the product.

A typical problem in extruding PEX pipes is that residues of peroxidecollect in the extrusion head and have to be removed on a daily basis.This problem can be overcome by providing non-crosslinked material onboth sides of the product. Considering drinking water quality aspecially beneficial alternative for the non-cross-linked inner materialis a polymer that is impermeable to residues which are formed in thecross-linked section of the product due to chemical reactions duringcross-linking.

In conventional orientation of polyolefines the molecule chains areelongated and stressed under influence of the stretching force. On theother hand this phenomenon is counter-balanced by so-called relaxation,which tends to restore the molecular chains to the coiled, disorderedcondition. In the process of the invention the cross ties orinterference between the chains prevent the extremely rapid relaxationso that the draw speed need not be so limited to obtain suitablybalanced values. However, the material to be oriented may be, aftercross-linking, at the processing temperature, in a glassy state andhence rather brittle. Hence the stretching rate should not be too high,because otherwise the melt may react elastically and break due to itsbrittleness. It has been found that polyolefin compositions with widemolecular weight distribution do not break so easily. Surprisingly, ithas been found that, when the material is suitably chosen, the skinlayers on the product greatly enhance the available stretching rates,and can carry the brittle layer without rupturing. The brittleness ofthe cross-linked layer itself can also be improved by a careful choiceof the molecular weight distribution of the polymeric material, or bythe use of additives known in the art which improve melt strength.

On similar grounds a process that does not rely on stretching too muchis preferred.

                  TABLE I                                                         ______________________________________                                        Degree of cross-linking %                                                                          Increase in tensile                                      strength at          break %                                                  ______________________________________                                        22                    75                                                      33                    88                                                      60                   116                                                      87                   128                                                      ______________________________________                                    

Table I above illustrates the improvement obtained by the method of theinvention. The right hand column indicates the increase in tensilestrength at break for PEX samples cross-linked and uniaxially stretched100% at 170° C. during orientation of the material compared tocross-linked, non-stretched samples. The table shows the permanentdifference in strength of the samples as a function of the degree ofcross-linking. It also shows that achieving permanent orientation andenhanced strength properties at high draw temperatures is most unlikelyto be achieved unless the molecules are tied, for example, bycross-linking before drawing.

In a further example, when a 0.8mm thick PEX sample is cross-linked to80% and drawn, at a temperature of 200 deg.C., to an elongation of 500%,a tensile strength of 182 Mpa is obtained. In many experiments, it hasbeen determined that the tensile strength of the oriented material is alinear function of the draw rate.

In the above examples, the density of the uncross-linked PE raw materialis 955 kg/M³. The density of a cross-linked (70% gel content) sample ofthe same PE is 929 kg/m³. The corresponding density of a cross-linkedand oriented sample is 938 kg/m³. Thus it can be seen that the method ofthe invention provides products having a higher density than thoseproduced without orientation of the polymeric material.

The invention is particularly applicable to the production of relativelythick walled pipes, especially those wherein the ratio of wall thicknessto diameter is at least 1:100, preferably greater than 2:100, morepreferably greater than 3:100.

The dimensions of plastics pressure pipes and vessels are determinedusing the hydrostatic design base established by long term pressureresistance data and regression analysis. Normal HDPE grades have adesign base of 6.3 MPa and the very best contemporary high molecularweight PEs have a design base (MRS) of 10 MPa. The tests shown in Table1 above were made with cross-linked PE having typically a design base of8. Oriented pipe samples of the same material produced in accordancewith the invention can have a design base of at least 12 Mpa up to 16Mpa or higher.

One of the problems encountered when designing high performance plasticspipes for pressure and pressure sewage use is that even if the highallowed sigma value (allowed long term stress in the wall), which is thebase for dimensioning the pipe wall to withstand pressure, would allowrather cost effective pipes with relatively small wall thickness, thepipe itself fails in practice because of other restrictions. Forexample, if the sigma value is increased from today's 8 N/mm₂ (PE 100)to a level of 16 or 20, which is possible with orientation according tothe present invention, the wall thickness gets so thin that the ringstiffness of the pipe installed underground may cause the pipe to bucklewhen subjected to pressure surges. Although the modulus of the materialincreases somewhat because of orientation this is not enough tocompensate for the reduced wall thickness because the ring stiffnessfollows the third power of the wall. Although fillers like fibres etceffectively increase the modulus a more effective way is just toincrease the wall thickness. This becomes, however, expensive and a newmethod of making stiff oriented pressure pipes is required.

The above problem can readily be solved as described earlier in thisspecification by using a pipe wall having a multi-layer construction.This construction can have one or several oriented layers in the productwhich provide pressure resistance, a middle layer which consists ofplastic foam and an outside layer protecting the whole structure. It canbe made by extruding and orienting the whole structure. The inner layerwill be permanently oriented because the crosslinking agent incorporatedis activated. The middle layer consisting of, for example, polyethylene,together with a foaming agent which also begins to react because of theincreased temperature, forms a foamed layer around the pressure pipecore. The outer layer, which typically would be of softer, ductilematerial, follows the expansion during orientation and subsequentfoaming step and forms the outer protective layer which typically wouldalso contain all the necessary stabilisers, dyes etc.

The final pipe can also be coated, or provided with release agents andanother release layer, which subsequently can be peeled away.

Typical foaming degrees are up to 50% (of original middle layerdensity). But excellent pressure pipes with a very lightweight foam canalso be produced, with foam densities of less than 500 kg/m³, forexample, densities down to 30 kg/m₃. In this latter case the soft middlelayer also acts like an excellent cushion against disturbances causedafter pipe laying. In tests performed, foams containing simultaneouslyfibres or fibre like materials such as Wollastonite, appear to offerexceptionally good strength characteristics.

It is also possible to extrude oriented pipes having more than one foamlayer using the method of the invention. For example, a multilayer pipemay be extruded having two foam layers of different density. Multilayerpipes incorporating a metal layer and one or more foam layers can alsobe produced. Examples of such products include; a multilayer pipecomprising an oriented PEX inner layer, an adhesion layer which can befoamed, an intermediate metal layer, a second adhesion layer which canbe foamed, and a protective outer layer; and a multilayer pressurisedsewer pipe which comprises a thin oriented PEX inner layer, a firstintermediate layer comprising a rigid foam optionally including fillers,for example at least 10%, preferably about 25%, of calcium carbonate, toincrease its ring stiffness, a second intermediate layer comprising aprotective flexible foam, and a protective outer layer, preferablyincluding a UV stabiliser, which can be a further crack resistant PEXlayer.

Pipes having a thin oriented inner layer, a fibrous mineral filled foammiddle layer, and a cross-linked outer layer are particularly suitablefor use in sewer pipe applications. The cross-linked outer layer can beformed from a scratch resistant polymeric material which allows "nosand" installation, the middle layer can be sturdy with a relativelyhigh stiffness, and the inner layer can provide a pressure tolerantwaterway wall. A further application of such pipes can be in "no dig"installation methods wherein the pipe is pushed through the soil.

The present invention can also be used to produce a multilayer orientedplastics material pipe comprising an inner pipe and an outer pipeforming an inner layer and an outer layer, respectively, and betweensaid layers an intermediate layer of a softer material than the innerpipe. Such a pipe, and a method for its manufacture, are described andclaimed in our co-pending Finnish applications no. FI 955960 and 961822,the entire disclosures of which are incorporated herein by reference forall purposes.

It has also been surprisingly discovered that not only are the orientedproducts of the invention extremely strong, but that in many cases theclarity of the product is greatly improved. For example, withcross-linked polyethylene (PEX) totally transparent products can beformed which may find application for bottles and other uses. PEXproducts are not normally clear. Transparent, oriented, cross-linked PEarticles produced in accordance with the invention can find manyapplications because of the low permeability of the material. Bothcross-linking and orientation improve the diffusion properties of thematerial.

The invention facilitates joining of pipes having a spigot end and asocket end, which have been produced by the method of the invention. Asealing ring is mounted on the spigot end of one pipe and is located inthe intended position by a gripper, for example, a metal ring, or bydouble-sided sand paper wrapped around the pipe. The socket end of theother pipe is widened mechanically, and the spigot end with the sealingring is pushed into the socket. After a short time, for example, about15 seconds, the socket has returned to its original condition clampingthe sealing ring between the inside of the socket and the outside of thespigot with higher force than in normal PEX-pipes.

In one preferred embodiment of an apparatus according to the invention,the product is fabricated by melt extrusion of the polymer in anapparatus including an annular orifice having a diametrically diverginggeometry and (preferably but not essentially) converging walls andorifice area, whereby the polymer is substantially simultaneouslyelongated circumferentially and axially.

In understanding those embodiments of the present invention whereinorientation takes place within a closed die, two influencing factorsshould be borne in mind. Firstly, since relaxation of the orientedmolecules requires expansion in volume or in cross-section flow, itcannot easily occur within the extrusion die orifice of the apparatus ofthe invention due to the radial constraining action of the wallsthereof. However, as soon as the polymeric material exits from theoutlet end of the extrusion die, it is no longer subject to such radialconstraint, and any unsolidified oriented molecules will tend to relax,thereby causing radial swelling of the product, unless, as in thepresent invention, there is a thick enough rigid skin layer presentand/or polymer chain mobility is limited. Secondly, the closer theoriented molten polymeric material is to its melting point, the longeris the time necessary for relaxation to occur.

In another preferred embodiment of the invention, wherein the product isoriented in the hoop direction using a closed die, a haul off is onlyused to balance the properties of the product. This process is very easyto run compared to existing processes, and can produce continuousorientation of practically all thermoplastic polymeric materials, frombiopolymers and rubbers to engineering plastics.

In addition, the same principle can be used, for example, for themanufacture of oriented injection moulded parts with no weld lines, forthe production of oriented, fibre reinforced blow moulding parts, coatedcable structures or bi-oriented films or sheets, and for themanufacturing of thick walled sheets using calendaring techniques.

The oriented polymeric articles of the invention can be joined by anysuitable conventional technique, for example by the use of mechanicalfittings, heat shrinkable sleeves and fittings, and fusion techniques,including welding, and, especially, electrofusion fittings and joints.The method of the invention can also be used to produce orientedpolymeric pipe fittings, for example, by injection moulding. In aparticularly preferred embodiment, the invention provides for theproduction of oriented electrofusion pipe fittings by injection mouldingan oriented polymeric material around an electrofusion heating element.Examples of (unoriented) electrofusion pipe fittings which can beproduced by the method of the invention in oriented form are describedin EP 0591245, EP 0260014, EP 0243062, EP 0353912, EP 0189918, and WO95/07432, the entire disclosures of which are incorporated herein byreference for all purposes. Oriented electrofusion pipe fittingsaccording to the invention can be used to join unoriented plasticspipes, but find especial application in the joining of oriented pipeswhich have also been made using the method of the invention. Theadvantage of such oriented electrofusion pipe fittings is that they canbe much stronger than conventional unoriented fittings, and also thatthe pressure which is required to be developed during electrofusionjointing can be enhanced by the retraction (shrink) force which can begenerated by the tendency of the oriented polymeric material of the bodyof the fitting to recover when heated by the electrofusion heatingelement.

In the jointing of multilayer composite polymeric articles according tothe invention, for example, pipes having an intermediate metal layer,which, if unprotected, may be subject to corrosion, a novel method offabricating the pipe ends may be used. In this method, the outermostlayers of the pipe can be removed, preferably in the factory, and aninner layer of weldable polymeric material exposed. This inner layer canthen be folded back 180 over the pipe end to cover and protect the pipeend and to be welded against the outer wall of the pipe. In this way,the former inner layer of the pipe becomes the outermost layer, givesgood sealing against corrosion, and provides a good welding surface fornormal welding and jointing techniques such as electrofusion.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of apparatus according to the invention will now bedescribed in detail by way of example only with reference to theaccompanying Drawings in which:

FIG. 1 shows a first embodiment of a pipe extrusion line for practisingthe method of the invention in axial cross-sectional view;

FIG. 2 shows a similar view of another embodiment of a pipe extrusionline for practising the method of the invention;

FIG. 3 shows a similar view of a further embodiment of a pipe extrusionline for practising the method of the invention, and

FIG. 4 shows an axial cross-sectional view of an embodiment of aninjection moulding apparatus for practising the method of the invention.

FIGS. 5(a) and (b) show in diagrammatic form, two embodiments ofapparatus according to the invention for the manufacture of a compositemetal/plastics pipe.

FIG. 6 shows a mechanism to graft ricinoloxazoline maleinate (OXA) ontopolypropylene.

In the Drawings the extruder itself is not shown, although in most casesa conventional screw extruder can be used. Certain materials with veryhigh molecular weight may require ram extruders (piston extruders) orthe like instead of conventional screw extruders. Also multilayerproducts can be extruded by means of ram extruders by applying suitablecrosshead technology.

In FIGS. 1 to 3 and 5, the pipe is radially expanded using a mandrel,which preferably is supported at a first end by the extruder body, forexample, by using a support member extending through the extruder screw,and/or optionally at a second end by a calibrator, for example, whereinthe mandrel or a support member therefor rests on the solidified wall ofthe polymeric material extrudate passing through the calibrator.

It is important that at least the layer to be oriented preferably isextruded with a tooling system that is totally spider free, e.g. themandrel is supported upstream of the material flow and hence gives aflow without any weld lines. The need for this is due to brittleness ofmany glassy state cross-linked polymers. Any spiders in the flow ofmaterial that has begun to cross-link will have detrimental effects tothe hoop strength of the product, and this becomes clearly visible whentrying to expand the parison. Very effective in minimizing thedetrimental effects of weldlines are certain cross-heads with rotatingdie-sets. A rotating mandrel with counter rotating sleeve can also givea desired fibre orientation in the hoop direction where fibres are addedto the plastics material. Examples of suitable arrangements can befound, for example, in FI 83184, GB 2089717, GB 1325468, U.S. Pat. No.3,244,781, WO90/15706, WO84/04070, EP 057613, the entire disclosures ofwhich are incorporated herein by reference for all purposes.

(i) Orientation After Extruder Die

In FIG. 1 there is fragmentarily shown a die 10 and a fixed core 11which form part of an extrusion head of a conventional pipe extruder(ram or screw extruder) and define an annular die opening. The innercore projects from the extrusion head and forms at its free end amandrel 11A.

An olefin (co)polymer material, together with a suitable quantity ofcross-linking agent, is plasticized in the extruder and is dischargedfrom the end of the extruder as a cylindrical tubular extrudate 12having a relatively large wall thickness. At the discharge opening ofthe extrusion head there is provided a heater 10A such as a radiationheater, for heating the tubular extrudate to a temperature which issufficient in order to cross-link the material thereof to a degreeranging from 1 to 100%.

Downstream of heater 10A there are provided along the path of thetubular extrudate two opposite circulating trains of concatenated mouldhalves 13 which are moved in an endless path over drive sprockets 14.Along the path of the tubular extrudate the mould halves are guided bymeans, not shown, to come together at mandrel 11A and to form abipartite mould forming a cylindrical mould cavity enclosing the tubularextrudate. The mould halves are driven along the path of the tubularextrudate in the direction of movement thereof at the same speed as thatof the extrudate.

A mandrel 15 is located inside the tubular extrudate and is attached tothe extrusion head by means of a bar 16. Through a passage in the bar agaseous fluid such as air or an inert gas is supplied to the interior ofthe tubular extrudate in the space defined between mandrel 11A andmandrel 15 in order to keep the wall of the tubular extrudate engagedwith the surfaces of the bipartite mould cavities. The mould halves 13are heated at a suitable location in the endless circulation paththereof, e.g. at 17, by suitable heating means operating with fuelburners or electric resistance elements. When the extrudate wallcontacts the heated bipartite moulds heat will be supplied to thepolyethylene material to maintain said material at the cross-linkingtemperature for a period sufficient to reach the desired degree ofcross-linking.

Downstream of mandrel 15, a plug 18, preferably of a balloon type, isprovided in the tubular extrudate said plug being anchored to mandrel 15by means of a rod 19. Pressurized fluid is supplied to the balloon plugthrough passages in rods 16 and 19 to keep the plug inflated in sealingengagement with the inner surface of the tubular extrudate. In the spacebetween mandrel 15 and plug 18 a pressure is maintained by means offluid such as air or inert gas supplied to said space through passagesin rods 16 and 19 said pressure being higher than the pressuremaintained in the tubular extrudate between mandrel 11A and mandrel 15.The tubular extrudate, which is still soft, will be exposed to freeexpansion radially allowing hoop stretching of the wall thereof underthe influence of this higher pressure to form a tubular member with alarger diameter than that of the tubular extrudate leaving the extruder,and with a wall thickness that is reduced in relation to the wallthickness of said extrudate.

Outer support rollers 20, which may be connected to a drive mechanism toimprove the process control possibilities, are provided at mandrel 15 tosealingly engage the tubular member against said mandrel, and acalibrator 21 is provided in the path of the tubular member located in aposition where the pipe has been expanded. Calibrator 21 forms a passagedetermining the outer diameter of the finished tubular member andprovides cooling for the tubular member by the supply of cold waterwhich is distributed over the outer surface of the tubular memberthrough apertures 22 in the surface of the calibrator which is engagedby the moving tubular member. In a further embodiment, the calibratorcan be omitted and replaced by a conventional corrugator when acorrugated oriented pipe is to be produced.

The cooling of the tubular member is sufficient to solidify the polymermaterial so that the tubular member when exiting from the calibrator 21is a rigid pipe downstream of the calibrator. A take-up device 23 isprovided which engages the outside surface of the rigid pipe andoperates to impart to the pipe axial traction. The speed of the take-updevice preferably should be adjustable so that the positive tractionforce imparted to the moving pipe can be controlled. It should bementioned that the traction force could also in special cases benegative because the pipe gets shorter during expansion if not drawn.

After hoop stretching of the at least partly cross-linked olefin(co)polymer material by expansion of the tubular member between mandrel15 and calibrator 21 and the axial stretching of the pipe effected bytake-up device 23, the finished pipe should preferably have arelationship between wall thickness and diameter which is at least1:100, preferably around 2:100 or greater, for example greater than orequal to 3:100. The hoop stretching of the pipe material causesorientation of the olefin (co)polymer material in the hoop direction andthis stretching preferably should range from 25% to 400% and preferablyis around 100%. The axial stretching of the pipe material preferablyshould range from 0% to 400%, more preferably about 30%, and causesorientation of the olefin (co)polymer material in the axial direction.By the bi-directional orientation of the (co)polymer material improvedstrength is imparted to the pipe, and due to the (co)polymer materialbeing at least partly cross-linked when the orientation is effected,such orientation can be effected and maintained in a wide temperaturerange, typically from 135° C. to 250° C.

Further cross-linking can be effected after expansion of the tubularmember on the expanded pipe at a position between calibrator 21 andballoon plug 18. This can be effected, for example, by gamma radiationor electron radiation of the pipe, but preferably is obtained by heatingof the extruded pipe at said position, provided that there is asufficient amount of cross-linking agent left in the material after theinitial cross-linking obtained by heating of the (co)polymer material inthe bipartite moulds.

Such reheating can be effected by means of circulating trains of heatedconcatenated mould halves as previously described and subsequentcalibration and cooling between calibrator 21 and balloon plug 18. Thefurther cross-linking after orientation of the (co)polymer material canprovide increased dimensional stability against reversion of theorientation at higher temperatures.

Heating of the tubular member immediately downstream of the extrusionhead can be dispensed with if the (co)polymer material is heatedsufficiently in the extruder to be kept at the necessary temperature fora sufficient time for cross-linking to the desired degree to take placebefore orientation. It should also be understood that other means forholding the temperature of the extruded tubular member or for reheatingthe pipe, respectively, than heated circulating mould halves, e.g. aheating bath or dielectric heating, can be used. However, circulatingmould halves are preferred, for example, in manufacturing orientedribbed pipes.

Radial Expansion within the Die

Free radial expansion of the tubular member is used in the embodimentdescribed above, but the expansion can also be effected over a mandrelinside a jacket or similar device surrounding the tubular member asshown in FIG. 2.

The mandrel 11 is supported preferably through the extruder in order toavoid spider legs which leave weak spots in the material which begins tocross-link. The mandrel diameter is kept constant or increasescontinuously or stepwise until the final expansion starts at mandrelhead 11b.

It is important that the heat flow from the hot tooling 10, 11 isprevented from reaching the low temperature area of the apparatus,comprising the extruder and the die entry. If necessary, suitableinsulation should be provided. A typical temperature difference betweenthe extruder screw end of the apparatus and the hottest end part of thetooling is 50 deg.C. or more.

In the embodiment of FIG. 2 mandrel 11 is extended to form a mandrelhead 11B which widens conically in the flow direction of tubularextrudate 12 to expand said extrudate radially so as to stretch theplastics material in the hoop direction. The conical portion of mandrelhead 11B joins a cylindrical portion for inside calibration of the pipeformed by expansion of the tubular extrudate. The mandrel head thus hasa substantially S-shaped contour. Suitable angles of the conical partdepend on the extrusion speed. Suitable values range from 5 degrees upto 30 degrees. Greater angles can easily lead to too fast a deformationspeed which will cause the properties of the oriented article todeteriorate. Practical usable and preferred deformation speeds rangefrom 0,002 to 5 s-1. Die 10 is extended to form a jacket 10A enclosingthe tubular extrudate when passing from the extruder to and over theconical portion of mandrel head 11B. Thus, it will be seen that mandrelhead 11B and jacket 10A define a space for the radial expansion of thetubular extrudate passing therethrough. The surfaces defining said spacecan be coated with a low friction material such as for examplepolytetrafluoroethylene.

Radial Expansion Onto a Mandrel After the Die

In this embodiment, the jacket of FIG. 2 can end close to the pointwhere the conical portion begins. In this case speed controlled rollerscould be provided in the vicinity of mandrel head 11B. Jacket 10A can beprovided with electric heating elements on the outside thereof forheating the tubular extrudate as may be necessary in order to impart tosaid extrudate the temperature necessary for the desired cross-linkingto take place when the tubular extrudate is passing through the jacket.Further cross-linking in this case can easily be achieved by extendingthe heated length of mandrel head 11B. Furthermore, the end part ofjacket 10A can be cooled in order to give a shiny outside to the pipeand for the purpose of locking (preventing) excessive die swell. Alsomandrel head 11B can be heated over the conically widening portionthereof and can be cooled downstream of said portion with a series ofdifferent cooling circuits. Cooling is needed to freeze the orientationbut also because of the good surface finish it gives to the inside ofthe product. Throughout the process avoiding stick-slip flow is criticaland correct temperatures of the sliding surfaces are essential to keepthem slippery.

An optional bar 19 is connected to mandrel head 11B and anchors balloonplug 18 to the extruder, said head being located at the entrance end oftake-up device 23. As in the embodiment previously described there arein bar 19 passages for supplying a gaseous fluid such as air or inertgas under pressure to balloon plug 18 and to the interior of the pipeformed after expansion of the tubular member. Between mandrel head 11Band balloon plug 18 there are provided nozzles 24 for sprinkling coolingwater over the pipe both when it passes over the cylindrical portion ofmandrel head 11B and when it has left said portion in order to rigidifythe calibrated pipe.

The benefit of the mandrel process described that it can easily be usedfor both internally calibrated pipes (cooling extension of mandrel head11B) and for externally calibrated pipes (with a similar arrangement asin FIG. 1). The need for plug 18 depends partly also from thelubrication system. In a preferred embodiment the pressurized fluidbetween plug 18 and mandrel head 11B, which can be used for forcing thestill soft member against an outside calibrator, can function as alubricant, at least for the start-up phase, between the inside of themember and the outside of the mandrel and the plug, respectively.

The cross-linking process can be initiated already at the end of theextruder, for example, inside die 10, by any suitable means, e.g. UV, ifdie 10 is made of glass. Also, radiation or electron beam cross-linkingcan be used. Then, the main part of cross-linking is carried out in thedie or the bipartite moulds. The exact point where cooling of the memberstarts after expansion should be chosen with regard to the desiredcross-linking in the expanded state. A long hot section in the mandrelhead 11b serves the function of secondary cross-linking of the productin order to enhance its dimensional stability.

Known processes for stretching of plastic pipes, e.g. the processdisclosed in DE 23 57 210, most often include a fairly long conicalmandrel. In order to achieve high orientation rates shorter conicalparts might be interesting. On the other hand, if the orientation takesplace freely i.e. by means of a differential pressure over the wall ofthe tubular member, then said member can adopt an S-shaped curve, whichis illustrated in EP 0563721, where it is used for free expansion afterthe die, the cross-section being close to inverted hyperbolic orparabola shape. This shape is often seen in film blowing, and resultsfrom a balance of modulus, drawing speed, temperature, wall thicknessand draw ratio.

Surprisingly, this shape is also effective as a form of mandrel in theclosed die system of the present invention as illustrated in FIG. 3.

Without wishing to be bound by any particular theory, it is believedthat the optional hydraulic lubricating agent, which can be injected atboth sides of the tubular member, forms with this shape a natural, wellbalanced hydrodynamic cushion. The benefit of this form is that thelikelihood of the material dragging on the mandrel is reduced. This hasbeen found beneficial also where no lubricating agents are used butstable plug flow is achieved with coatings or by the use of internallubricants. The tendency of high molecular weight material to flow in aso-called "slip-stick" fashion should be minimised as far as possible.In an analysis of extruded pipes, it has been observed thatunsatisfactory pipes almost invariably show a flow pattern on theirsurfaces (not visible to the naked eye) which a Fourier transformanalysis reveals to have an amplitude of 0.8 mm or more. In satisfactorypipes, with a steady plug flow in the tooling, no such pattern is found.In these cases, coatings which have good lubricity properties, forexample, polytetrafluoroethylene, can be adequate. Low friction in thecavity area is important for the process to work. Very good results havebeen obtained using a cavity having a rough metal surface which has adiamond like surface (DLC), in which any irregularities have been filledwith Teflon.

Liquid coatings can be used but generally are of very limited endurance.Hydraulic lubricating agents, for example, silicone-oil or glycol canhowever give excellent results. Also, internal lubrication of thepolymer material can be effective. Suitable internal lubricants dependon the material to be processed but for example Acuflow (trade mark),fluorinated rubber compounds such as Viton (trade mark) and Dynamar(trade mark) can be used.

FIG. 3 shows an extrusion line wherein the polymeric material isoriented or aligned in the thermoplastic state prior to cross-linkingand final orientation.

A conical extruder 31, for example as described in EP 0422042, isillustrated schematically. This extruder can permit the support of amandrel through the extruder as preferred in certain embodiments of theinvention. In addition, the extruder can produce a multiple layeredproduct if desired. Other suitable extruders can, of course, be used asappropriate.

33a and 33b represent schematically different material feeds to theextruder, and 32 is a rotating double screw.

Through the extruder, a hollow shaft 42 is connected to a mandrel 41.Axial movement of the shaft can be adjusted by means of a nut 44.

The temperature of the material in the extruder is kept below thereaction (cross-linking) temperature up to the extruder orifice 43.

After the outlet 43 the polymeric material 34 enters a cavity means 35defined by the mandrel 41 and an outer jacket 48. In this section, thediameter of the mandrel 41 is increased in order to orientate themolecules of the polymeric material. Initially, however, the temperatureis still kept substantially below the reaction temperature.

At around the mid-point of the mandrel 41, or towards the end of itsconical section, the temperature of the polymeric material is raisedusing the heaters 46 around the surrounding outer jacket 48. Further, oralternatively, heaters may be positioned inside the mandrel 41 (notshown). Any suitable heating method may be used, for example the outerjacket may comprise sections of material transparent to IR or RFradiation from suitable heating sources. In this heated section thereaction begins. The reaction time can be determined by the length of acylindrical second part of the mandrel 41a. In some cases thecylindrical part 41a can be omitted or replaced by a section having asmoothly increasing or stepwise increasing diameter.

The polymeric material exits the discharge orifice 35a of the cavitymeans and enters the inlet end 36a of the die orifice 36b of theextruder die 36.

The extruder die 36 contains the final orientation mandrel 45, which isconnected to the mandrel 41 and is also heated. The mandrel 45 has asmoothly increasing diameter, and a curved, substantially parabolicouter surface as shown. Alternatively, the entire conical die could alsobe smoothly conical with increasing diameter from flow area A1 to flowarea A3, for example, having a cone angle of from about 3 to 30 degrees.

The extruder die can also optionally be provided with temperaturecontrol means for maintaining in the die orifice an axial temperaturegradient descending in the direction of flow through a median dietemperature substantially equal to the normal melting point of thepolymeric material so that solidification of the polymeric material willbe inhibited in the inlet end 36a of the die orifice and may beinitiated within the die orifice 36b, for example, towards the dieoutlet 37.

The heated mandrel 45 is connected to a cooling mandrel 47 which gives asmooth inner wall to the extrudate and also freezes in the orientationproduced in the polymeric material. For a similar purpose, the outerjacket 48 is provided with short cooling rings 49 at the die outlet 37.

In the example, the flow area A1 at the extruder outlet is substantiallythe same as flow areas A2 at the cavity means discharge orifice and A3at the die outlet, and this configuration, in which there issubstantially no increase in the cross-sectional area of flow, ispreferred. In certain cases, however, the areas A2 and A3 can be smallerthan A1. In general, the flow areas A2 and A3 are from 0.9 to 2.0 timesthe area of A1. Preferably the arrangement is such that the orientedpolymeric material is constrained against its natural tendency to loseits molecular orientation by radial swelling.

When the polymeric material leaves the die outlet 37 (at A3) there maystill be some cross-linking proceeding. This can be beneficial as it canreduce any tendency to shrink back.

After leaving the die outlet 37, the extruded polymeric pipe contactsthe cooling mandrel 47 and from the cooling mandrel, the polymeric pipeenters a calibration sleeve 50. Within the calibration sleeve 50, or inthe vicinity thereof, the pipe can be supported with a balloon type plug(not shown) for the purpose of inducing fluid pressure against thecalibration sleeve. In order to reduce friction against the calibrationsleeve wall water lubrication can be used. The calibration sleeve itselfcan have a serrated internal surface which can be coated, for example,with a friction reducing coating such as Teflon or diamond.

The haul off and cooling tanks of the apparatus are of conventionaldesign and are not shown in the Drawing.

Orientation During Injection Moulding

FIG. 4 shows on example of a suitable set up for the injection mouldingof cross-linked oriented plastics pipe bends. The injection mouldingapparatus 60 comprises a body 66, surrounding a mandrel 61, 62, 63, inthree sections. The first section 61 of the mandrel provides thenon-oriented inner dimension of the pipe bend. The second section 62 isa heated conical section whereby the plastics material is radiallyexpanded and oriented. The third section is a heated cylindrical sectionwhereby further cross-linking of the plastics material can take place.The plastics material resides in passage 64, between the body 66 and thefirst section 61, and is conveyed by the action of the extruder screw(not shown) to the passage 67, via the conical passage 62a, between thebody 66 and the mandrel section 62, in which it is oriented andcross-linked. The oriented and cross-linked plastics material receivedin the passage 67 is then forced by the action of the sleeve piston 65(shown in its retracted position) into the injection mould 70. The mouldhas an "end gate" type opening 72 into the mould cavity 68, and has acore 69. As illustrated the mould also has a pipe bend socket section71, which can be provided with a collapsible core (not shown).

A similar apparatus can be provided, in accordance with the invention,for producing an oriented blow moulded product. In this case, themandrel section 63, the core 69 and the collapsible core of the socketsection 71 can be replaced by a pressurisable fluid.

Examples of further extrusion apparatus and articles produced therebywhich can advantageously be used in and produced by the method of thepresent invention are described and claimed in our copendingInternational patent applications nos. PCT/FI96/00261 andPCT/FI96/00359, the entire disclosures of which are incorporated hereinby reference for all purposes.

In FIG. 5(a) and (b) there are shown in fragmentary diagrammatic crosssection two devices for the manufacture of a metal/plastics compositepipe. In FIG. 5(a) an extruded cross-linkable parison 80 issuing from anextruder die outlet 81 is pressed against a metal pipe 82 by means of aconical heated mandrel 83. The heated mandrel raises the temperature ofthe parison to the cross-linking temperature and at the same timeorients the plastics material of the parison by giving it a diametraldraw. The metal pipe is formed by helically winding a metal strip andwelding or mechanically interlocking the lateral edges 84 of the strip.The metal pipe can be transported at the same speed as the extrusionspeed, or faster, if it is desired to impart an axial draw to theplastics material.

FIG. 5(b) shows an alternative device in which an oriented plastics pipe90 is formed by extruding a cross-linkable parison of plastics materialfrom an extruder die 91, and cross-linking and simultaneously impartinga diametral draw to the plastics material of the parison by means of aconical heated mandrel 92. An outer metal or fibre reinforced plasticssleeve 94 is formed over the oriented plastics pipe by helically windinga strip 93 of metal or (fibre reinforced) plastics material of anysuitable cross-section therearound.

Materials

The crystalline or semi-crystalline thermoplastic polymeric material canbe, for example, an olefin (co)polymer which throughout thisspecification includes olefin homopolymers, copolymers or melt blends oftwo or more (co)polymers which either inherently or as a consequence ofmelt blending have the desired haul-off-tension, molecular weight andmolecular weight distribution characteristics. Preferably the olefin(co)polymer to be extruded should have a density which is at least 900kg/m3, more preferably above 920 and most preferably from 930 to 960kg/m3. The definition of polyethylene in this context includescopolymers of ethylene with at most 5% by weight of an alkene-1 with 3or more carbon atoms. In a preferred embodiment as described below thematerial is HD polyethylene with the addition of organic peroxides ascross-linking agents for cross-linking during extrusion, and phenolicantioxidants.

Preferably the additions of peroxides and antioxidants are each in totalfrom 0.1-1.5% by weight of the polymeric material, preferably 0.3-0.5%.

Generally, the material to be crosslinked or vulcanized can be anycrosslinkable extrudable material such as polyolefins, ethylenecopolymers, vinyl polymers, polyamides, polyesters, polyurethanes,fluorinated polymers or co-polymers, and elastomers, in particularethylene-propylene elastomers and some synthetic rubber compounds.Preferably the orientable crystalline or semi-crystalline thermoplasticpolymeric material is a semi-crystalline polymer such as polyethylene,polypropylene or polyvinylidene fluoride, an amorphous crystallizingpolymer such as polymethylmethacrylate or a crystallisable polymer suchas polyvinylchloride, polyesters or polycarbonates. The startingmaterials can be in granulate or powdery form.

Useful polymers or comonomers which can be blended with the orientablethermoplastic polymeric material matrix (especially a polyolefin matrix)prior to extrusion in order to improve the properties of the orientedend product include, for example, ethylene vinyl acetate,EPDM-terpolymers, polybutadienes, copolymers of isobutylenes withconjugated dienes, mono- and polyfunctional acrylates and methacrylates,paraffin waxes, maleinates, especially ricinoloxazoline maleinate (OXA),maleinanhydride, styrene etc.

Typical cross-linking agents are different peroxides such as dicumylperoxide and certain dimethacrylates and azo compounds. Also silanes canbe used as cross-linking agents for cross-linking of material sectionsof the finished product in a water oven. For the outside of the productalso cross-linking by irradiation or photoinitialized systems areavailable. Whichever cross-linking process is used, it may beadvantageous to incorporate one or more co-curing agents for examplepolyunsaturated monomers such as triallyl cyanurate, diallyl phthalate,benzoquinone and ethylene glycol dimethacrylate. The cross-linking agentis preferably added to the polymeric material in an amount of at least0.01% by weight, more preferably from 0.1 to 5% by weight, mostpreferably from 0.1 to 1.5% by weight, for example from 0.3 to 0.5% byweight.

By the addition of fillers such as fibres or flakes (e.g. mica) in thecross-linked layers and in the non cross-linked layers or in some layersonly, for example, the heat deflection temperature (HDT) of the productcan be increased. Any suitable discontinuous fibre may be used. Fibreswhich reinforce matrices generally include fibres having an averageaspect ratio of 10-3000. Various types of organic and inorganic fibresare suitable either in monofilament or stranded form. Illustrativeexamples of satisfactory discontinuous fibres include polyamide, rayon,polyester, glass, asbestos, stainless steel, carbon, wollastonite andceramic whiskers. Typical loading levels are from 10 to 30%.

Examples of useful laminar fillers include mica, talc and graphiteflakes. Chalk, silica and fly ash may also be included. The amount offiller or fibre which may advantageously be included depends on thenature of the filler, but up to 50% may usefully be incorporated.Especially useful fillers are, for example, those that make the polymerconductive such as carbon black, react to dielectric heating methodssuch as induction or microwave heating or are (ferro)magnetic by nature.

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiments. The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

We claim:
 1. A method of forming and continuously orienting an articlecomprising a crystalline or semi-crystalline thermoplastic polymericmaterial(s) at a temperature greater than the crystalline meltingtemperature of raid material(s), which comprises the steps of:adding achemically reactive substance(s) to the polymeric material before orduring forming of either the entire article, or to one or more layers ofa multilayer article, or to axial or helical stripes of the article, orto certain segments of the article in the axial direction; plasticizingand forming a parison of the polymeric material(s) thus prepared at atemperature not high enough to activate the reaction of the saidreactive substance(s); optionally, inducing shear at least to thelayer(s) where the chemically reactive substance(s) have been added toand/or stretching the still soft parison in one or both of twodirections, simultaneously or stepwise, said stretching including axialdraw to effect thermoplastic orientation of the material in thelongitudinal direction of the parison and/or radial expansion to effectthermoplastic orientation of the material in the hoop direction of theparison; decreasing the mobility of the molecules in the layer(s) to beoriented by activating a chemical reaction between the chemicallyreactive substance(s) and the polymeric material(s) having thechemically reactive substance(s) added thereto when the polymericmaterial(s) still is/are in a molten state; inducing shear at least tothe product, layer(s), stripes, or segments to which the chemicallyreactive substance(s) have been added and/or stretching the still soft,at least partly reacted parison in two directions, simultaneously orstepwise, said stretching comprising axial draw to effect orientation ofthe material in the longitudinal direction of the parison and radialdraw to effect orientation of the material in the transverse directionof the parison; calibrating and cooling the parison in the orientedcondition to make the orientation permanent at least in the layer(s)where the chemical reaction(s) has taken place.
 2. The method of claim1, in which the said chemically reactive substance(s) when activatedcauses a reaction with the polymeric material(s) so that the newmolecules have decreased mobility in the molten state because of sterichindrance.
 3. The method of claim 1, in which the said chemicallyreactive substances comprises a cross-linking agent which is able tocross-link the polymer molecule chains.
 4. The method of claim 1, inwhich the reaction needed for getting the material in permanentlyorientable state is activated with additional heat or radiation afterinitial thermoplastic orientation.
 5. The method of claim 1, in whichthe mobility of the molecular chain is reduced at least in one of thelayers close to or in the one to be oriented by the addition ofinorganic or organic flake minerals or fibers or such material whichwill fibrillate during orientation.
 6. The method of claim 1, in whichthe melt strength or the parison, needed for the axial and radial draw,is improved by the addition to one or more parts of the product oforganic and/or inorganic fillers, for example, fibres or flake shapedminerals, which can also be oriented and which act like nucleatingagents for the oriented crystallites if blended to the material in theoriented layer.
 7. The method of claim 1, in which at least to thelayers close to the or each layer to be oriented fibres and/or mineralshave been added which respond to dielectric heating and this heating isused to rapidly increase the temperature in the layer containing thereactive substance(s) for the activation of the reaction(s).
 8. Themethod of claim 1, in which at least to the layers not to be orientedorganic and/or inorganic fibres or flake shaped minerals up to 10-50% byweight have been added and/or said layer is further cross-linked, withthe same cross-linking method as during drawing or by another method,during the process and after it has reached its final dimensions to anadditional gel content level of up to 80%.
 9. The method of claim 1, inwhich the parison is first stretched in a thermoplastic state in orderto get the molecular chains structured and substantially aligned andthen again during/after the reaction phase, and this total draw, atleast in the axial direction, is up to 600%.
 10. The method of claim 1,in which the parison is extruded as a tubular member having arelationship between wall thickness and diameter which is greater than2:100 and the axial and hoop orientations are in the same or differentlayers of the product.
 11. The method or claim 1, in which the tubularmember is exposed after initial orientation inside an extrusion tool tofree axial and or radial expansion which is facilitated by maintaining adifferential fluid pressure over the wall of the tubular member.
 12. Themethod of claim 1, in which the article is blow moulded, and afterleaving a die set, the parison is further stretched and oriented byblowing it into a cavity and the internal overpressure is fed into thecavity through an extruder.
 13. The method of claim 1, in which thearticle is injection moulded.
 14. The method of claim 1, in which theparison is pressed during further cross-linking against mould(s), whichcan be moving with said member, and said moulds are heated for holdingthe material at a cross-linking temperature.
 15. The method of claim 1,in which the orientation takes place within a closed die.
 16. The methodof claim 1, in which the parison is exposed to radial expansion on acontinuously or stepwise enlarging mandrel by drawing the parison overthe mandrel.
 17. The method of claim 1, in which the parison is enclosedby a jacket which is either heated or cooled and the material of theparison pushed with extrusion pressure through a die cavity, defined bythe said mandrel and the said jacket, and the material is radially andaxially oriented within the die cavity and a further axial draw isoptionally applied to the enlarged extrudate exiting the die cavity. 18.The method of claim 1, in which the parison to cooled internally withina die set with an integral cooling mandrel, of substantially the samediameter as the stretching mandrel, and which may extend out of the dieset.
 19. The method of claim 1, in which the mobility of the moleculechains is reduced and hence the die swell is decreased by immediateslight cooling of the parison, coming out of a die set, with a waterspray or an air flow before entering a calibration sleeve.
 20. Themethod of claim 1, in which the extent to which the product iscross-linked and oriented is partly controlled by choosing the startingpoint of cooling in relation to the point where the final dimension ofthe product is reached, for example, by closing or activating coolingmedia circuits coupled to the jacket and mandrel of claim
 17. 21. Themethod of claim 1, in which the plastics material to be crosslinked is apolyolefin composition comprising a higher melt flow rate olefin polymeror co-polymer having an average molecular weight (Mw) ranging from30,000 to 1,000,000 g/mol and a lower melt flow rate olefin polymer orco-polymer having a molecular weight greater than 600,000 g/mol, wherethe difference in viscosity is, at least ten fold.
 22. A methodaccording to claim 1, wherein the orientation is carried out at atemperature within the range of from 135° C. to 250° C.
 23. A methodaccording to claim 1, for the production of wire and cable coatings. 24.A method for the production of an oriented crystalline orsemi-crystalline thermoplastic polymeric article which comprises:(i)heating A crystalline or semi-crystalline thermoplastic polymericmaterial to a temperature at or above--its crystalline melting point;(ii) forming the polymeric material into an article whilst at atemperature at or above its crystalline--melting point; (iii) subjectingthe polymeric material to shear forces and/or stretching, either duringor after the formation of the article to effect orientation of thepolymeric material in the longitudinal and transverse directions; (iv)reacting the polymeric material either before, during, or after theformation of the article, and either before or during orientation, orafter orientation but before substantial relaxation of the orientationhas taken place, with a crosslinking agent, or a grafting agent wherebystearic hindrance of polymer chain movement is increased;the resultantarticle having a tensile strength in the directions of orientationgreater than the tensile strength of an unoriented article formed fromthe polymeric material.
 25. The method according to claim 24, whereinthe article le an elongate hollow article.
 26. The method according toclaim 24, wherein the article is a hollow tubular article formed byextrusion.
 27. The method according to claim 24, wherein the polymericmaterial is mixed with the crosslinking agent or grafting agent withinan extruder.
 28. The method according to claim 24, wherein the articlehas a wall thickness of greater than 0.8 mm, preferably greater than 2mm.
 29. The method according to claim 24, wherein the article is atubular article and orientation is effected in both the longitudinal andthe hoop directions.
 30. The method according to claim 24, wherein thethermoplastic crystalline polymeric material is reacted with thecrosslinking agent or the grafting agent in a first stage and thensubjected to shear forces and/or stretching to effect orientation of thematerial in a second stage.
 31. The method according to claim 24,wherein the crystalline thermoplastic polymeric material is subjected toshear forces and/or stretching to effect orientation of the material ina first stage and then reacted with the crosslinking agent or thegrafting agent in a second stage before substantial relaxation of theorientation has taken place.
 32. The method according to claim 24,wherein the crystalline thermoplastic polymeric material is subjectedsimultaneously to shear forces and/or stretching to effect orientationof the material and to crosslinking or grafting to increase sterichindrance of polymer chain movement.
 33. The method of claim 24, whereinthe crystalline thermoplastic polymeric material is subjected to radialexpansion to effect orientation of the material in the hoop direction.34. The method according to claim 24, wherein the crystallinethermoplastic polymeric material is reacted with the crosslinking agentor the grafting agent in an extruder, or in an extruder die, the degreeof crosslinking of the polymeric material at the point where theextrudate leaves the die being at least 2%.
 35. A method for theproduction of a multilayer tubular article comprising an orientedcrystalline or semi-crystalline thermoplastic polymeric article whichcomprises:forming a first material into tubular form by axially foldingor helically wrapping a sheet of the first material, and then lining thefirst outer parison thus formed with a single or multilayer second innerparison comprising a crystalline or semi-crystalline polymeric material,wherein the said polymeric material is subjected to shear force and/orstretching to effect orientation of the polymeric material in thelongitudinal and/or directions, and urged at a temperature at or aboveits crystalline melting point into contact with the inner surface of thefirst parison using a conical tool whilst substantially maintaining theorientation of the said polymeric material.
 36. The method according toclaim 35, wherein an outer layer of the inner parison is formed from anadhesion plastics material, preferably grafted PE, which comprises afoaming agent, and this outer layer is allowed to foam at least to sucha degree that when an oriented inner layer of the inner parison shrinksto its diameter at ambient temperature the foamed outer layer fills thecavity formed between the inner surface of the outer parison and theouter surface of the said inner layer.
 37. The method of claim 35,wherein the said foamed outer layer also comprises one or more fillerswhereby the modulus of the foamed layer is increased such that when thesaid inner layer of the inner parison is subjected to pressure, theinner layer is supported by the outer parison through the foamed outerlayer.
 38. The method of claim 35, wherein the first material comprisesa metal sheet or strip.
 39. The method of claim 35, wherein thepolymeric material is extruded into contact with the first parison andthe first outer parison is transported at a speed higher than theextrusion speed, whereby the polymeric material coming into contact withthe first parison is subjected to an axial draw and orientation.
 40. Anarticle comprising a crystalline or semi-crystalline polymeric material,in which at least part of the product is cross-linked or has graftedside chains or end groups creating steric hinderance and its permanentlybiaxially oriented at ambient temperature, the article having a tensilestrength in the directions of orientation greater than the tensilestrength of an unoriented article formed from the same polymericmaterial.
 41. The Article of claim 40, that is a hollow elongatearticle.
 42. The article of claim 33 or 41, in which said part forms oneor more stripes along the axis of the product preferably in helicalform.
 43. The article of claim 40 in which said part forms concentriclayers around the axis of the product.
 44. The article of claim 41, inwhich the article has a wall which comprises at least two layers whichare crosslinked by different methods and which show differing degrees oforientation.
 45. The article of any of claim 33 to 44, in which thearticle forms a hollow geometric profile having a relationship betweenwall thickness and averages diameter which is greater than 1:100,preferably greater than 2:100.
 46. The article of claim 40, in which thearticle comprises an oriented, cross-linked structural layer made orpolyethylene having a pressure resistance at ambient temperature equalto a hydrostatic design base of at least 12 Mpa, preferably at least 16Mpa.
 47. The article of claim 40, in which the oriented and crosslinkedpart or parts make up more than half the volume of the article.
 48. Thearticle or claim 46, in which the article has an outer skin of plasticmaterial which is substantially non-oriented, the thickness of said skinbeing 0.01 to 3 mm and having a high permeability.
 49. The article ofclaim 46 or 48, in which the article has an inner skin of plasticsmaterial which is substantially non-oriented, said skin having athickness of 0.01 to 10 mm and comprising a non-crosslinked layer havingbarrier properties different from those of the oriented and crosslinkedlayer(s) and preferably being impermeable to bi-products generated inthe chemical reaction, e.g. cross-linking of other layers of theproduct.
 50. The article of claim 46, in which a non-oriented part orparts and an oriented part or parts are made of the same polymericmaterial.
 51. The article of claim 40, in which the plastics material ofthe oriented and crosslinked part or parts comprises a polyolefincomposition comprising an olefin polymer or (co)polymer having anaverage molecular weight (Mw) ranging from 30.000 to 1.000.000, and anolefin polymer or (co)polymer having a molecular weight greater than600,000 g/mol.
 52. The article of claim 40, in which one or more partsof the article contain discontinuous fibres or flakes, which are alsooriented.
 53. The article of claims 41, in which the article is amultilayer bi-oriented article wherein in at least one layer of thearticle there is an interlacing orientation field, wherein the polymericmaterial is helically oriented or directed to form a reinforcing netlike structure into the hollow article.
 54. The article of claim 53, inwhich the interlacing orientation field comprises oriented liquidcrystal plastics and/or crosslinked, oriented fibre like polyethylenemolecule chains.
 55. The article of claim 40, characterized in that whenheated to a temperature above its crystalline melting point it shrinksless than would be predicted from its draw ratio.
 56. The article ofclaim 40, in which the density of the oriented layer is higher than thedensity of said layer in its unoriented state.
 57. The article of claim40, in which the article comprises a pipe wherein at least one of thelayers is also foamed and preferably cross-linked.
 58. The article ofclaim 40, which comprises a metal layer.
 59. The article of claim 58, inwhich the metal layer comprises a pipe or tube formed by folding orwinding a metal sheet or strip.
 60. The article at claim 58, whichcomprises an inner layer of an oriented polymeric material.
 61. Thearticle of claim 58, in which the article comprises an outer metallayer, an intermediate foamed adhesion layer and an inner orientedpolymeric layer.
 62. The article of claim 40, which comprises across-linked oriented pipe bond.
 63. The article of claim 44, in whichthe article comprises a multilayer pipe comprising an oriented thickwalled inner layer, a foam intermediate layer and a protective outerlayer.
 64. The article of claim 63, in which the foam density is below500 kg/m³ and the ring stiffness of the outer layer is lower than thatof the inner layer.
 65. A composite tubular article comprising a coil,or a coiled sheet or strip of metal having a thickness of from 0.2 mm to5 mm, and an extruded tubular polymeric material arranged in one or morelayers, the article having improved strength properties and at leastpart of the polymeric material being both cross-linked and permanentlyoriented at ambient temperature.
 66. The composite article of claim 65,in which the article is a hollow article and in which outside the metallayer there is provided a layer of foamed polymeric material ofthickness from 1 to 100 mm which provides both insulation and mechanicalprotection.
 67. An extrusion apparatus for producing an orientedextruders of polymeric material, comprising;a) a plasticating extrudermeans for providing a melt(s) or partial melt(s) of said polymeric(s)material(s) and a chemically reactive substance and feeding said meltunder pressure through a discharge opening in said extruder means; b) anelongational flow pattern-developing cavity means having an inletopening communicating with said discharge opening of said extrudermeans, a flow cavity, and a discharge orifice, the relative geometriesof said flow cavity and said discharge orifice being such as to developwithin the molten polymer material flowing from said extruder meansthrough said cavity means an elongational flow pattern which inducesmolecular orientation at least transversely to the direction of flowwithin said molten polymeric material; c) an orientation-retainingextrusion die means provided with an orifice extending therethrough,said die orifice having an inlet end and outlet end, said dischargeorifice of said cavity means opening into said inlet end of said dieorifice so as to permit flow of the oriented molten polymeric materialfrom said cavity means into said die means, said discharge orificehaving a cross-sectional area in the range of from 0.9 to 2.0 times thecross sectional area of said die orifice; d) temperature control meansfor maintaining the temperature of said flowing molten polymericmaterial below the reaction temperature of the said chemically reactivesubstance in the extruder and in at least a first part of said cavitymeans, and for maintaining the temperature of said flowing moltenpolymeric material above said reaction temperature in at least a secondpart of said cavity means and/or said inlet end of said die orifice; e)optionally, temperature control means for maintaining in said diporifice an axial temperature gradient descending in the direction offlow through a median die temperature substantially equal to the normalmelting point of said polymeric material so that solidification of saidpolymeric material will be inhibited in the entrance region of said diemeans and may be initiated within said die means; and f) optionally, avariable spend take-up means for withdrawing an extrudate of saidpolymeric material from said outlet end of said die orifice at acontrolled draw rate;the arrangement being such that said extrudatebegins to solidify within said die means or after exiting from saidoutlet end of said die orifice but before any substantial radialswelling of said extradite can occur.
 68. The extrusion apparatus ofclaim 67, wherein the die orifice has diametrically diverging geometryand converging walls and orifice area, whereby the polymer material issubstantially simultaneously elongated circumferentially and axially.69. The extrusion apparatus of claim 67, in which the cavity meanscomprises a mandrel which is fixed in such a manner that thecross-section of the flow cavity is kept substantially constant from thescrew end of the extruder to the point where stretching of the plasticsmaterial starts.
 70. The extrusion apparatus of claim 69, in which themandrel is supported by the body of the extruder through the screwand/or optionally, through the solidified wall of the polymeric materialextrudate, by the calibrator.
 71. The extrusion apparatus of claim 67,in which the flow passage is free of obstacles capable of forming weldlines in the extrudate at least in the heated regions wherein thetemperature is above the reaction temperature.
 72. The extrusionapparatus of claim 67, in which the diameter of the mandrel issubstantially constant from the extruder outlet to the point wherestretching starts, and, optionally, at its other extremity issubstantially constant from the point at which the extrudate begins tosolidify up to the calibrator.
 73. The extrusion apparatus of claim 67,in which the mandrel forms a conically widening portion.
 74. Theextrusion apparatus of claim 73, in which an outer jacket extends atleast partly over said conically widening portion.
 75. The extrusionapparatus of claim 74, in which the mandrel is heated over a portion,including said conically widening portion, and downstream thereof iscooled.
 76. A method of producing a pressure pipe, the methodcomprising:forming said pipe of a material comprising an at leastpartially cross-linked crystalline or semi-crystalline thermoplasticpolymeric material, which is biaxially oriented.
 77. The methodaccording to claim 76, wherein the polymeric material is a polyolefin.78. The method according to claim 77, wherein the polyolefin ispolyethylene.
 79. The method according to claim 76, wherein the pressurepipe is of multilayered construction, at least one of the layerscomprising an at least partially cross-linked biaxially orientedcrystalline and semi-crystalline thermoplastic polymeric material. 80.The method according to claim 76, wherein the orientation is effected ata temperature within the range of from 135° C. to 250° C.
 81. The methodaccording to claim 76, wherein the polymeric material is oriented in thehoop direction by stretching by from 25% to 400%, and in the axialdirection by stretching by up to 400%.
 82. A composite tubular articlecomprising a coil, or a coiled sheet or strip of metal having athickness of from 0.2 mm to 0.5 mm, and an extruded tubular polymericmaterial arranged in one or more layers, the article having improvedstrength properties and at least part of the polymeric material beingboth cross-linked and permanently oriented at ambient temperature,wherein the article is a hollow article and in which the ring stiffnessof the polymeric material layer is sufficiently high with respect toring stiffness of the metal layer such that when the hollow article isdeformed and the deformation stress is removed, the hollow articlerecovers at least partially elastically to its original form.
 83. Acomposite tubular article comprising a coil, or a coiled sheet or stripof metal having a thickness of from 0.2 mm to 0.5 mm, and an extrudedtubular polymeric material arranged in one or more layers, the articlehaving improved strength properties and at least part of the polymericmaterial being both cross-linked and permanently oriented at ambienttemperature, wherein the article is an electrofusion pipe fitting, theshoot, strip, or coil of metal comprises an electrofusion heatingelement, and the fitting has a body comprising the oriented,cross-linked polymeric material.
 84. A method of producing a pressurepipe, the method comprising:forming the pipe of a material comprising anat least partially cross-linked crystalline or semi-crystallinethermoplastic polymeric material, wherein the polymeric material isbiaxially oriented; and further cross-linking the polymeric materialafter orientation.